1. Field of the Invention
The present invention relates to a method and device for forming reinforcement elements composed of resistant fibers distributed along three dimensions. It also relates to the composite pieces obtained from said reinforcement elements after impregnation thereof with a hardenable binder, such as a synthetic resin.
2. Description of the Prior Art
From the American patents U.S. Pat. No. 2,283,802 and U.S. Pat. No. 3,322,868 a method is already known for example for forming composite elements from fibers distributed along three dimensions. In these known techniques, a substrate is formed from such fibers crossed in at least two directions, then the fibers of said substrate are bound together by stitching, by introducing a continuous fiber in said substrate using a needle driven with a reciprocal movement, whereas a relative movement of said needle with respect to said substrate occurs, so that said continuous fiber forms a zig-zag stitching line inside said substrate.
The use of such techniques raises no difficulty, when the two faces of the substrate are free, for then said needle may penetrate into said substrate by one of the faces and leave through the opposite face. On the other hand, when said substrate rest on a support, it is necessary to treat said support with special care.
For example, in the American patent U.S. Pat. No. 4,080,915 in which said support is rigid, openings are formed therein for allowing the end of the needle to penetrate therein without breaking. It will be noted that such a solution greatly limits the possibilities of stitching, since the number of openings formed in the support is necessarily limited and that it is imperative, for each stitch, for said needle to be opposite such an opening. Thus, in this prior patent, the openings are formed of elongate slits and the stitching lines are rectilinear, in correspondence with said slits. It can then be seen that not only the density of the stitches on the substrate must necessarily be low, but in addition, the form of the stitching lines and the distribution of the stitches are imposed once and for all by said openings.
On the other hand, in the American patent U.S. Pat. No. 4,863,660, the support is formed from a foam material or similar, allowing penetration of the point of a needle. In this case, there is no longer a limit imposed on the density of the stitches or the form of the stitching lines, but it is necessary to make the support from an easily destructible material, so as to be able to release the substrate from said support.